The present invention relates to a process for the preparation of calcium cyanamide by nitrifying calcium carbide in rotary furnaces.
The nitrification of technical calcium carbide in a rotary furnace with the addition of calcium cyanamide and small amounts of a calcium halide, such as, for example, calcium fluoride or calcium chloride, is well known. See, for example, Ullmann's Enzyklopadie der technischen Chemie ["Ullmann's Encyclopedia of Industrial Chemistry"], Volume 9 (1975) pages 85-95. In the Trostberg rotary furnace process which is used industrially, the calcium carbide, together with about 25-30% by weight of calcium cyanamide and 0.5-1% by weight of fluorspar, is about 90% ground to less than 0.09 mm in tube mills; the ground material formed is blown by means of nitrogen into an expanded rotary furnace port and is reacted with nitrogen at 1000.degree.-1150.degree. C. However, it is not possible to prevent severe caking of the reaction material on the walls of the rotary furnace following this procedure. A considerable manual effort is therefore required to scrape these cakings off the rotary furnace wall. This results in considerable expense relating to personnel and repairs, and appreciable losses in the nitrification yield also have to be accepted.
Attempts have been made to prevent severe caking by using a controlled narrow carbide grain size distribution of 0.075-3 mm (for which several comminution and screening stages are necessary), and by adding 1-2% by weight of calcium chloride, by graduated heat conductivity in the lining of the rotary furnace and by specially controlling the metering in of nitrogen using the Knapsack process. However, the process has not proved successful because of the considerable expense and also the calcium chloride content, which interferes with the subsequent production of calcium cyanamide derivatives.
The use of briquettes of finely ground carbide for nitrification has also already been suggested. This suggestion has also not found acceptance in the industry because of the additional expense required (the briquette press, transporting equipment for briquettes and intermediate storage for and screening of the briquettes) and because of the residual carbide which occurs in the interior of the briquettes as a consequence of excessively high densification (in order to achieve transportable briquettes resistant to breaking).